Linear predictive coding (LPC) is one of the more important tools used in the processing of voice information. LPC is a mathematical procedure for estimating a filter function equivalent to the vocal tract. The estimate of the vocal tract resonance may be used to subtract vocal tract resonances from speech leaving an estimate of the excitation. The vocal tract function is estimated by removing correlation between a number of adjacent samples of the speech waveform; assuming that the waveform may be modeled as exponentially decaying sinusoids. The model for decaying sinusoids may be derived by inverting a correlation matrix (an all pole lattice digital filter) to provide an all zero lattice digital filter. The LPC correlation, excitation, and amplitude information are each individually quantized and transmitted typically at between 1200 and 4800 bits per second depending on desired speech fidelity, system complexity, and system throughput constraints. Typical apparatus for providing the LPC correlation, excitation, and amplitude information is disclosed in a copending application entitled "Human Voice Analyzing Apparatus", filed of even date herewith and assigned to the same assignee.
The quantized LPC correlation, excitation, and amplitude information is supplied to a voice synthesizer which synthesizes or reconstructs the voice from the quantized information. The speech synthesis can be performed by any of several different methods including the all pole lattice filter method (basically the inverse of the all zero voice analysis method), cascaded second order filter, direct form filter, pole and zero filter, etc. Prior art synthesizers have the disadvantage of being limited to a specific type of voice synthesis and, in general, are limited to a very narrow type of applications. That is, prior art synthesizers which are constructed on a single semiconductor chip are generally not capable of full fidelity reproduction of a human voice.